The present invention relates to a color image forming apparatus represented by a color electronic copying machine, a color printer and the like using an electrophotographic method, which layers single-color images to form a color image.
U.S. Pat. No. 5,087,945 discloses an image forming apparatus which includes four image forming sections and two transfer drums. Color-component images formed by the four image forming sections are layered and outputted to a transfer material by one of the transfer drums.
In the image forming section apparatus disclosed in this U.S. Patent, a first image is formed on one transfer drum. A second image is formed on the same transfer drum at a timing at which the first image is conveyed by rotation of the transfer drum, and thus, first and second images are layered. The layered images are transferred to the other transfer drum. Thereafter, third and fourth images are sequentially formed thereon at timings at which the layered images are conveyed by rotation of the transfer drum, thereby to layer the images on the same transfer drum.
In the image forming apparatus described above, duet to the directions in which two intermediate transfer drums 5 are rotated and the feature of the layout of four image forming units 1 to 4 faced to the drums, for example, an image formed by the first image forming unit 1 is turned once on an intermediate transfer drum 5 which rotates in the clockwise direction, and is thereafter turned substantially three or four times on an intermediate transfer drum 5 which rotates in the anticlockwise direction. The image is then conveyed to a transfer position faced to the roller 7. Therefore, there is a problem that the time until images of four colors are layered and form one color image is elongated.
In addition, since the position of the image forming unit for forming a black (monochrome) image is node clearly defined, there is also a problem that the first copy time required until image formation of the first image after an instruction of the image formation is elongated.
The length of the first copy time is equal both in the case of forming a black image of single color and the case of forming a color image by layering four color images. Particularly when forming one sheet of single black image, the waiting time is long.
In addition, from another proposal not shown in the figures, four photosensitive drums are provided as image carriers in many color image forming apparatuses, and a charger, an exposure device, a developing device, a transfer device, a cleaning device, and a discharger are provided orderly around each of the photosensitive drums. Each of the first to fourth image forming units for forming predetermined colors is constructed by those devices provided along the circumference of the photosensitive drum.
The first to fourth image forming units are a set of total four units, i.e., an image forming unit for yellow, an image forming unit for magenta, an image forming unit for cyan, and an image forming unit for black.
In each of the first to fourth image forming units, a well-known electrostatic photographic process in the order of charge, exposure, development, transfer, cleaning, to discharge is carried out on the surface of the photosensitive drum. Meanwhile, a paper sheet as a transfer material is let pass through the units sequentially, so visible images of predetermined (four) colors formed by the image forming units are transferred one after another. Further, the images of multiple colors are fixed finally by a fixing device not shown which is provided next to the fourth image forming unit.
In this kind of color image forming apparatus, toner images in four colors formed by four image forming units are sequentially layered in case of forming an image of a predetermined color. Also, in this kind of color image forming apparatus, images are superposed at most four times.
Further another proposal for a well-known color image forming apparatus shows a method as follows. Toner images formed by four toner image forming sections for forming images of three colors respectively corresponding to the color components of subtractive primaries and a black image for strengthening the black density are formed on a belt-like photosensitive surface for every color, and are layered on an intermediate transfer drum. Thereafter, images of four colors are transferred onto a paper sheet by a transfer device.
However, in the color image forming apparatus of this kind, the total length of the intermediate drum is set to the maximum length of the image to be printed out, so there is a problem that the diameter is increased.
The object of the present invention is to provide a high-speed color image forming apparatus which is capable of time required for image formation when forming a color image by layering a plurality of toner images and particularly capable of reducing time required for copying in single black color.
The present invention has been made in view of the problems described above and provides an image forming apparatus for forming four charged toner images, comprising: four toner image forming sections; first and second intermediate transfer members; four transfer devices for transferring four toner images formed by the four toner image forming sections to the first and second intermediate transfer members; a between-intermediate-transfer-member transfer member for transferring the four toner images transferred to the first and second intermediate transfer members, between the first and second intermediate transfer members; and an output transfer device for transferring, onto an output transfer material by the between-intermediate-transfer-members transfer member, a toner image obtained by layering and transferring a toner image, which is obtained by positioning and transferring one or several of the toner images formed by a corresponding number of the toner image forming sections to the first intermediate transfer member, onto a toner image, which is obtained by positioning and transferring one or several of the toner images formed by a corresponding number of the toner image forming sections to the second intermediate transfer member, wherein, where FP1 is a position at which the four toner images on the between-intermediate-transfer-members transfer member are transferred to the output transfer material, FP2 is a transfer position most distant from the FP1 among positions on the first and second intermediate transfer members at which the toner images are transferred from the toner image forming sections, a length of a course distance between the FP1 and FP2 is L+|FP1xe2x88x92FP2|, and a circumferential length of an image carrier is PL, 1.48 less than |FP1xe2x88x92FP2|/PL less than 2.25 is satisfied.
Also, the present invention provides a color image forming apparatus comprising: n sets of toner image forming sections for forming charged toner images of plural colors; two intermediate transfer members; a transfer device for transferring a plurality of charged toner images formed by the toner image forming sections, to the intermediate transfer members; a between-intermediate-transfer-members transfer member for transferring the toner images transferred to the intermediate transfer members, between the intermediate transfer members; a second transfer device for transferring, to the between-intermediate-transfer-members transfer member, a toner image from the toner image forming section, which has been transferred to at least one of the intermediate transfer members; a detection device provided near each of the intermediate transfer members and the between-intermediate-transfer-members transfer member, for detecting a position of each of toner images transferred to the intermediate transfer members and the between-intermediate-transfer-members transfer member; a toner image position adjustment device for adjusting positions of the toner images; and a transfer material transfer device for transferring a toner image transferred to the between-intermediate-transfer-members transfer member or the second intermediate transfer member, to an output medium, wherein, if each of the intermediate transfer members corresponds to two or more of the toner image forming sections, relative positions of two or more toner images which are formed by the transfer devices on the first or second intermediate transfer member and which should be positionally aligned with each other in a first step are detected, and the position of the toner image formed by the toner image forming section is adjusted by the toner image position adjustment device, such that a difference caused between the relative positions is minimized on the first or second intermediate transfer member, as a first step, and relative positions of two superposed toner images which are transferred to the second intermediate transfer member by the between-intermediate-transfer-members transfer device and which should be positionally aligned with each other are detected by the detection device, the positions of the toner images formed on the second intermediate transfer member by the toner image forming sections are adjusted by the toner image position adjustment device, such that a difference between the relative positions of the superposed toner images is minimized, and a toner images obtained by superposing all of the toner images is transferred to an output transfer material by the transfer member transfer device.
Further, the present invention provides another color image forming apparatus comprising: four toner image forming sections for forming images of three colors respectively corresponding to color components of subtractive primaries and a black image for strengthening a black density; first and second intermediate transfer media to which two of four images of single colors formed by the four toner image forming sections are transferred; four first transfer devices for transferring the images of single colors respectively formed by the image forming sections to the first and second intermediate transfer members; a third intermediate transfer medium for layering images, which are respectively on the first and second intermediate transfer media and are each obtained by layering two of the images of single colors transferred by the four first transfer devices; two second transfer devices for transferring the images layered by the third first and second intermediate transfer media, to the third transfer medium; and a third transfer device for transferring a color image thus layered on the third intermediate transfer medium, to an output transfer material.